1. Field Of The Invention
The present invention relates to mobile radio telephone systems and, more particularly, to a method and apparatus for locating a mobile telephone within a wide geographic area covered by a mobile telephone service network.
2. History Of The Prior Art
The goal of a mobile telephone system is the interconnection of mobile telephone users ("mobile subscribers" or "mobile units") with the extensive public switched telephone network ("PSTN"). Successful achievement of this goal requires that each mobile subscriber in the system be made available to anyone who has a telephone, whether fixed or mobile. Hence, the problem of locating a mobile subscriber moving from one area to another (a "roaming subscriber") within a wide geographic area has become of primary importance.
Solutions to this problem are based upon the concept of mobile registration. Mobile registration is the process by which a mobile telephone unit becomes listed as being present in the service area of one of the mobile exchanges in a mobile telephone service network. As each mobile telephone unit enters a new area within the network, it transmits a unique identity signal which is detected by the mobile exchange associated with that area.
In one variation of the solution, the receiving exchange records an indication of the presence of the mobile unit in its memory and then informs all the other exchanges of the presence of the mobile unit within its coverage area at that particular moment. When the mobile unit crosses over into another area, the exchange associated with that area, upon receiving an identity signal from the telephone unit, will record an indication of the mobile unit's presence there and then transmit the identity signal to all of the other exchanges together with its own identity signal, for the purpose of updating the mobile unit's position.
In another variation of the solution, a mobile unit's identity and position messages are sent by each exchange, whose respective areas is crossed by such unit, to a specific center. Any exchange in the mobile network which contacts this center may receive all the information necessary for locating and making a connection to the mobile unit. This solution eliminates the need to advise one or more of the other mobile exchanges each time a mobile unit enters a new area without making or receiving a call there and thereby reduces the amount of mobile unit location data that must be processed by each of the mobile exchanges within the mobile network. In some systems, the aforementioned center may be a common national center such as that used in the mobile telephone location system disclosed in U.S. Pat. No. 4,700,374 issued to Bini. In other systems, the center may be the exchange to which a mobile unit is assigned (the "home" exchange). In such other systems, the mobile subscriber may preregister in an area other than the home area (i.e. the normal service and billing area) for service to be provided in the other ("visited") area. When a roaming subscriber arrives in the visited area, the mobile unit is qualified to make telephone calls from there and calls which are received in the mobile unit's home area are forwarded to the visited area for transmission to the mobile subscriber. Qualification of a mobile unit in a visited area may be automatically performed when the roaming subscriber appears in the visited area and the mobile unit is switched on, e.g. when the user initiates a first telephone call. The roaming mobile unit automatically transmits its identification number to the visited exchange and requests roamer service. If the roaming subscriber is a visitor from a cooperating exchange, the visited exchange provides service to the roaming subscriber by allocating a temporary roamer number to it. The visited exchange also notifies the roaming subscriber's home exchange of the roaming subscriber's location in the coverage area of the visited exchange. The roaming subscriber's identification number is then entered into a list of roamers in the home exchange so that incoming calls to the roaming subscriber are forwarded to the visited exchange where the roaming subscriber is then located.
Heretofore, proper routing of telephone calls to mobile subscribers has required that each mobile subscriber be registered exclusively in one of the local areas. Whenever a mobile subscriber moved from one local area to another, its registration in the local area from which the mobile subscriber had just departed was automatically cancelled and a new registration was established in the local area in which the mobile subscriber had just arrived. The list of roamers in the roaming subscriber's home exchange was then updated to reflect its new location. The home exchange, therefore, kept track of only one location, i.e. the visited area in which the mobile subscriber most recently registered, for each roaming subscriber at any particular moment.
While the single location concept underlying prior art roaming subscriber registration systems is consistent with physical reality, it does not fully take into account some of the limitations of radio technology. Often in prior art systems, telephone calls to a roaming subscriber were lost because the mobile subscriber was registered in one area but was actually located in another area. For example, a registration access signal sent by a roaming subscriber actually located in one visited exchange may be overheard by one or more other neighboring exchanges resulting in a plurality of cooperating exchanges registering the subscriber as a visitor and further resulting in incorrect location data updating in the home exchange of the roaming subscriber. In addition, a mobile may, in the processing of registering, rescan and inadvertently accesses a control channel associated with an exchange which is different from the one in which it is actually located. After registering on that channel the mobile again rescans and may retune to a control channel which is not in the same exchange in which it has just registered. However, the actual physical location of the mobile is unknown to the home exchange which has recorded the location from where the earlier registration was made. Because prior are systems automatically routed incoming calls to only the registered location of the roaming subscriber, for any call in which the registered location of the subscriber was incorrect the call was not completed. In addition, system resources were unnecessarily occupied whenever the call was not completed. The present invention dramatically increases the probability that a call will be completed to a subscriber even though its registered location was incorrect. This also minimizes the unnecessary waste of PSTN and mobile system resources. The known prior art fails to teach or suggest a multi-exchange paging system as set forth in the description of the present invention herein. For example, U.S. Pat. No. 4,843,622 issued to Yotsutani, et al. discloses a communication control unit capable of searching for a called telephone set in a mobile radio telephone network covering an area divided into a plurality of zones, each having a radio communication device assigned thereto. The specific searching operation classifies the zones into at least two groups so that at least one of the groups consists of a plurality of the zones. Searching proceeds by successively transmitting the incoming call signal to each group of zones until a response signal is received.
The Yotsutani approach apparently presumes a coverage area that is within the limits of a factory or a corporate headquarters and a location information signal for a specific telephone set which is static in the memory of the control unit. The efficiency of the searching operation disclosed by Yotsutani is a function of the size and number of the groups of zones which must be searched before a response signal is received. Hence, as the coverage area expands and the number of zones increases commensurate therewith, the efficiency of the searching operation decreases dramatically. In a wide area mobile telephone system providing for dynamic registration of roaming subscribers, searching one local area after another for each subscriber would strain the capacity of the network and would defeat the purpose of mobile registration.